You would be absolutely bonkers to authorise this repair and risk being hit with a hefty bill.
If they have asked Nissan U.K. for permission to remove the box then this permission should be immanent.
Why not leave the car with them pending warranty authorisation and take advantage of one of there courtesy cars.
I am only up in Perth and would only be to happy to diagnose the issues for you but that wont change the fact the the Nissan garage will still require authorisation from Nissan U.K. to remove the box as this is Nissan standard procedure.
Alex.